The DBMS usually uses an index in order to speed up retrieval. Especially the B-Tree structure and its extended structure are data structures suitable for a disk unit, and therefore are widely used as the most common index structure. In the B-Tree structured index is obtained by sorting a specific one or plurality of columns in a table in advance. The index is arranged so that search efficiency is increased, whereby search with the-column being used as a key is speeded up.
Meanwhile, as the amount of data has increased in recent years, attentions have been attracted to the cluster DBMS that uses a plurality of DB servers. As one of methods for operating the cluster DBMS, there is a method in which partition is performed with the hash-division algorithm or the range division method using a specific one or plurality of partition key columns in a table so that the same row in the table is not shared by the DB servers.
The cluster DBMS being operated by the above-mentioned method comes with a disadvantage that it will take a time to alter its configuration, such as a change of the number of DB servers. With the cluster DBMS, in the case of query processing that requires search results of all the DB servers to be summarized, query processing performance is governed by a DB server that takes the largest time. Since occurrence of a difference in the amount of data among the servers generally degrades the performance, it is essential to equalize the amount of data when altering the configuration. However, this processing requires data on a disk unit to be transferred to a physically different place, therefore taking a long time to do so.
U.S. Pat. No. 5,551,027 gives description about a hierarchical index for narrowing retrieval by a non-partition key in a cluster environment of the non-shared type DBMS. According to it, by using a global index consisting of a pair of a non-partition key and server number that maintains its column, it is possible to narrow down a server to be used for retrieval. However, the global index has only that capacity, but not a capacity for indicating a specific column in the table.